This invention relates to the field of High Voltage Direct Current (HVDC) transmission line apparatus and more specifically to a method and apparatus to resume normal operation of a high-voltage d.c. (HVDC) transmission line for power transmission between two a.c. systems.
In normal operation a first converter connected to the first a.c. system, NA, in a first station impresses a d.c. current into the HVDC transmission line, i.e., rectifier operation, while a converter in a second station connected to the second a.c. system takes the d.c. current from the HVDC transmission line and feeds it into the second system, NB, and thereby determined the d.c. voltage level of the HVDC transmission line converter inverter operation.
The inductance of the HVDC transmission line coupling the two converters thereby has the effect that any change of the voltage in the second station effects a disturbance or fault indication voltage for the current impression of the first station by a lag time determined by the transmission time or travel time of the HVDC transmission line, while any change in the current in the second station effects a change of the commutation times of the second converter with said lag time. Thus, an inverter shutdown of the second converter, in particular, occurs if, e.g., when resuming the power transmission, the HVDC transmission line current is run up rapidly by the run-up of the control angle in the first station and the control of the inverter is not reduced accordingly.
An inverter shutdown with a short-circuit of the HVDC transmission line voltage also occurs in the second station when the a.c. voltage collapses, thus interrupting the power transmission. A malfunction of normal operation with an interruption of the HVDC transmission line current also occurs in the first station in the event of a short circuit in the system or in the converter.
It is, however, economically important that in the event of said malfunction, during which both stations have to be disconnected or switched to emergency operation, normal operation is resumed as rapidly as possible. This requires that the previously malfunctioning station give a release signal to the continuously functioning station indicating the end of the malfunctioning as rapidly as possible. Both stations must then resume normal operation in a coordinated manner due to the mutual coupling via the HVDC transmission line, taking the transmission and processing times into account.
One method for the resumption of normal operation requires that both stations be synchronized taking the travel time of the synchronization signals into account. By that method normal power transmission cannot be approximately achieved until after a relatively long starting time.
It is an object of this invention to provide a method and apparatus for rapid resumption of normal line transmission.